Reciprocating plunger pumps are well known in the prior art. There are a number of controlling variables associated with the operation of such pumps. These variables include the area of the plunger, stroke length of the plunger (the distance that the plunger travels), the force exerted by the power unit in reciprocating of the plunger, and the number of times the plunger completes a stroke within a given time period. Because the area of the plunger is constant and may not be altered, this factor dictates one maximum pressure which may be obtained when the power unit exerts its maximum force on the plunger. The volume output per stroke is controlled by the area of the plunger times the distance that the plunger travels. The number of strokes per period of time determines the flow of the pump fluid. Although the pressure and volume of the pump fluid can be varied, the variation is limited due to the constant area of the plunger. One method of varying the pressure is by controlling the force exerted by the power unit on the plunger. In order to vary volume, the number of strokes per unit time can be varied by adjusting the speed control of the power unit. The volume output can also be controlled by adjusting the stroke length of the plunger. However, despite these adjustments, large pressure and volume variations cannot be achieved.
There has been disclosed in U.S. Pat. No. 2,047,167 (Heller) an adjustable clearance piston which is used to adjust the clearance space in order to vary the capacity of a compressor. A variable volume clearance device is also disclosed in U.S. Pat. No. 3,045,892 (White). This patent also discloses two shafts which can be coaxially connected.